1. Field of the Invention
The present invention relates generally to a technology for fabricating an optical fiber or an optical device, and more particularly to a method of fabricating an optical fiber or an optical device doped with reduced metal ion(s) and/or rare earth ion(s).
2. Background Art
An optical fiber containing metal ion and/or rare earth ion is brought under a special optical fiber, since it can be variously applied to an optical amplifier or an optical switching device etc. Therefore, much of the research in this area has been performed.
One of the research projects is a technique of reducing doped metal ion and/or rare earth ion, Generally, an atom has a different energy level distribution depending on its valence, and therefore has different spectroscopic characteristics such as light absorption and light emission. Accordingly, a little more diverse light absorption and light emission can be obtained by utilizing the change of valence and thereby the optical fiber and the optical device having various optical amplification and optical switching characteristics can be obtained.
As an example, let us consider rare earth ions, when a rare earth ion has the valence of 3+, the light absorption characteristic due to electronic transition between 4 f electron orbit and 5 d electron orbit occurs only in an ultraviolet wavelength region, whereas when the valence of the rare earth ion changes to 2+ ion, such a light absorption characteristic occurs in both visible and infrared wavelength regions. For this reason, a technique of making doped metal ion and rare earth ion with desired valences, respectively, is required. Furthermore, every atom has its own valence states in which the atom is mainly existed in nature and thus a specific process is required in order to transfer the valence into another valence.
For example, most of the rare earth ions have the valence of 3+. In order to stably transfer the valence of 3+ into the valence of 2+, 1+ or 0, it is necessary to reduce the rare earth ions. There have been proposed various reduction treatment methods as described below.
Firstly, there is a method of applying gamma rays to the rare earth metal ion having the valence of 3+. For example, it is reported that Tm2+ can be obtained, if the gamma rays is applied to a CaF2 crystal containing Tm3+.
However, in this method, there is a problem that a gamma ray source is dangerous to handle and the cost required in handling it safely is thus expensive.
Secondly, there is another method in which an aerosol type material is utilized. In this method, a MCVD (modified chemical vapor deposition) process is indispensable. In other words, this method includes the MCVD process in which a glass layer containing rare earth ions is deposited in a quartz glass tube, using material having aerosol formulation which generates carbon, together with a powder which generates rare earth ion and glass when fired. Then, processes of removing the carbon and OH radical, sintering the glass and collapsing the glass tube are, in turn, performed to thus obtain an optical fiber preform. For example, in a glass optical fiber having SiO2—Al2O3 components, Eu2+ and Sm2+ are reduced from Eu3+ and Sm3+, respectively.
To date, this method which utilizes the material having aerosol formulation is performed through only the MCVD process. A desired rare earth ion material having aerosol formulation and an additional apparatus for supplying material having aerosol formulation are needed.
Further, there is a method of injecting a mixture of H2 and Ar gases and obtaining the reduced rare earth ion during melting of glass. For example, in a glass having SiO2—Al2O3 components or SiO2—B2O3 components, Sm2+ is reduced from Sm3+.
In this method, there is a problem that processes of fabricating the optical fiber preform are complicated in comparison with the conventional processes and are not yet commercialized